Light emitting device driver circuit

ABSTRACT

A driver circuit is designed employing a current sensing circuit adapted to sense driving current being supplied to one or more light emitting devices. The driver circuit additionally employs a pulse width modulation circuit adapted to modulate a pulse width of a control signal based at least in part on an output of the current sensing circuit, the control signal being employed to adjust the driving current being supplied to the one or more light emitting devices. The driver circuit additionally employs a buck converter adapted to switch the driving current being supplied to the one or more light emitting devices.

FIELD

Disclosed embodiments of the present invention relate to the field ofimage projection, and more particularly to the employment of solid statelight emitting devices as illumination sources in the field of imageprojection.

BACKGROUND OF THE INVENTION

A number of projection systems designed to render images, or morespecifically, an image frame, by successively turning on and offselected ones of a number of solid state light emitting devices havebeen proposed. In a projection system, such use of light emittingdevices as illumination sources presents different design challengesthan the use of a projector lamp as an illumination source. The lightemitting devices are typically pulsed on and off in a rapid manner,whereas projector lamps typically remain constantly on throughoutoperation. Light emitting devices, such as light emitting diodes,typically exhibit a high sensitivity to spurious fluctuations in drivingcurrent. This high sensitivity to spurious fluctuations in drivingcurrent may result in unintended light output from the light emittingdevice, resulting in artifacts in the images ultimately rendered by theprojection system.

The many design challenges of using light emitting devices in aprojection system, only a few of which have been briefly discussed, willrequire skilled design of light emitting device driver circuits. As theuse of light emitting devices, such as light emitting diodes, continuesto expand, driver circuits for light emitting devices in other demandingapplications will likely make use of driver circuits designed for thedemanding application of image projection.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described by way of theaccompanying drawings in which like references denote similar elements,and in which:

FIG. 1 is a block diagram illustrating a number of functional blocks ofa light emitting device driver circuit, in accordance with an embodimentof this invention;

FIG. 2 illustrates some elements of a driver circuit for a lightemitting diode, in accordance with an embodiment of this invention;

FIG. 3 illustrates some elements of a driver circuit for a lightemitting diode, in accordance with an embodiment of this invention; and

FIG. 4 illustrates a projection system that uses a light emitting devicebased illumination source driven by a light emitting device drivercircuit, in accordance with an embodiment of this invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention include but are not limited tocircuits for driving light emitting devices, such as light emittingdiodes or laser diodes, used as illumination sources in a projectionsystem. The following discussion is primarily presented in the contextof light emitting diodes. It is understood that the principles describedherein may apply to other light emitting devices.

In the following description, various aspects of embodiments of thepresent invention will be described. However, it will be apparent tothose skilled in the art that other embodiments may be practiced withonly some or all of the described aspects. For purposes of explanation,specific numbers, materials and configurations are set forth in order toprovide a thorough understanding of the embodiments. However, it will beapparent to one skilled in the art that other embodiments may bepracticed without the specific details. In other instances, well-knownfeatures are omitted or simplified in order not to obscure thedescription.

Various operations will be described as multiple discrete operations inturn, in a manner that is most helpful in understanding the embodiments,however, the order of description should not be construed as to implythat these operations are necessarily order dependent. In particular,these operations need not be performed in the order of presentation.

The phrase “in one embodiment” is used repeatedly. The phrase generallydoes not refer to the same embodiment, however, it may. The terms“comprising,” “having” and “including” are synonymous, unless thecontext dictates otherwise.

FIG. 1 illustrates a block diagram of a number of functional blocks of alight emitting device driver circuit, in accordance with one embodiment.As illustrated, driver circuit 100 may include power processing block104. Power processing block 104 may be electrically coupled to lightemitting device 110. Power processing block 104 may receive input power102 and may transmit output power 108 to light emitting device 110.Light emitting device 110 may comprise one or more light emittingdevices, such as light emitting diodes. As will be described further inthe accompanying detailed embodiments, power processing block 104 maymake use of a buck converter to switch the driving current beingsupplied to the one or more light emitting devices. In variousembodiments, power processing block 104 may be capable of driving lightemitting device 110 with greater than or equal to approximately oneampere. In various other embodiments, power processing block 104 may becapable of driving light emitting device 110 with less thanapproximately one ampere.

Power processing block 104 may be electrically coupled to light emittingdevice 110 via transmission line 106. Current sensing circuit 112 may beelectrically coupled to transmission line 106 in such a way, examples ofwhich are detailed in subsequent figures, as to enable measurement ofdriving current being supplied by power processing block 104 to lightemitting device 110. Error amplifier circuit 114 may be electricallycoupled to both current sensing circuit 112 and voltage referencecircuit 116. Current sensing circuit 112 may supply a first voltage toerror amplifier circuit 114, and the first voltage may be related to thedriving current being supplied to light emitting device 110. Erroramplifier circuit 114 may compare the first voltage with a secondvoltage supplied by voltage reference circuit 116, and the secondvoltage may be used to specify the amplitude of the desired drivingcurrent being supplied to light emitting device 110.

Error amplifier circuit 114 may function to null the difference betweenthe first voltage and the second voltage by altering the first voltagevia feedback loop 122. Feedback loop 122 may include power processingblock 104 electrically coupled to current sensing circuit 112, currentsensing circuit 112 electrically coupled to error amplifier circuit 114,error amplifier circuit 114 electrically coupled to pulse widthmodulation circuit 118, and pulse width modulation circuit 118electrically coupled to power processing block 104. Error amplifiercircuit 114 may direct pulse width modulation circuit 118 to change thepulse width of the control signal being supplied to power processingblock 104, the pulse width of the control signal having a relationshipwith the amplitude of the driving current being supplied by powerprocessing block 104 to light emitting device 110. Power processingblock 104 may in turn change the amplitude of the driving current beingsupplied to light emitting device 110 to conform to the desired drivingcurrent. Current sensing circuit 112 may in turn sense said drivingcurrent, and may supply the first voltage based on said driving currentto error amplifier circuit 114. Feedback loop 122 may thus continuouslyoperate to control the driving current being supplied to light emittingdevice 110.

In various embodiments, the voltage supplied by voltage referencecircuit 116 may be fixed, fixing the desired driving current to besupplied to light emitting device 110. In various other embodiments, thevoltage supplied by voltage reference circuit 116 may be variable,allowing the specified driving current being supplied to light emittingdevice 110 to be changed.

Clock circuit 120 may be electrically coupled to pulse width modulationcircuit 118, and may transmit a clock signal to pulse width modulationcircuit 118. The clock signal frequency may determine the frequency ofthe signal supplied by pulse width modulation circuit 118 to powerprocessing block 104, which may in turn determine the frequency at whichpower processing block 104 drives light emitting device 110.

FIG. 2 illustrates some elements of a light emitting diode drivercircuit, in accordance with one embodiment. As illustrated, lightemitting diode circuit 200 may receive input power from voltage supplyVbulk 202. In some embodiments, power processing block 104 may includebuck converter 244. Buck converter 244 may include Schottky diode 242,MOSFET 238, and inductor 240. While buck converter 244 includes aSchottky diode, various other embodiments may include other types ofdiodes and may include more than one diode. While buck converter 244includes a diode, other embodiments may not include a diode. While buckconverter 244 includes a MOSFET, other embodiments may include adifferent mechanism for implementing a switch.

In various embodiments, current sensing circuit 112 may include currentsensing resistor 212, and differential amplifier 246. Differentialamplifier 246 may include operational amplifier 220, and resistors 214216 218 222 224, arranged as shown. Values of resistors 214 216 218 222224 are application dependent, and may vary from implementation toimplementation. Differential amplifier 246 may provide someamplification by operational amplifier 220 having a gain greater thanone. Current sensing circuit 112 may include a gain stage, which mayinclude operational amplifier 230 and resistors 226 228, arranged asshown. Values of resistors 226 228 are application dependent, and mayvary from implementation to implementation.

Pulse width modulation controller 232 may be an integrated circuitincluding error amplifier circuit 114, voltage reference circuit 116,pulse width modulation circuit 118, and clock circuit 120 of FIG. 1.Pulse width modulation controller 232 may include external components asmay be specified by the manufacturer or otherwise known in the art tofacilitate its operation.

Bipolar junction transistor 234 may act as a switch controlling timingfor the pulsation rate of light emitting diodes 204 206 208. The inputsignal to bipolar junction transistor 234 may be represented as LightBank 236, and may be generated by a timing circuit as practiced by thoseskilled in the art. Capacitor 210 may act to reduce ripple current inthe driving current being supplied to light emitting diodes 204 206 208.While the embodiment in FIG. 2 shows three light emitting diodes coupledin series, other embodiments may include a greater or lesser number oflight emitting diodes, serially or non-serially coupled together.

Pulse width modulation controller 232 may be electrically coupled toMOSFET 238, and may supply a control signal to MOSFET 238. The pulsewidth of the control signal supplied to MOSFET 238 may be at leastpartially modulated to compensate for the ripple sensed in the drivingcurrent being supplied to the one or more light emitting devices. Invarious embodiments, the ripple may include at least approximately fiftykilohertz of oscillation. In various embodiments, the ripple may fallwithin the range of approximately between twenty kilohertz and onemegahertz.

Light emitting diode driver circuit 200 may be capable of turning itsone or more light emitting diodes from an on-state to an off-state orfrom an off-state to an on-state, including settling at the desiredamplitude, in the time range of approximately 25 to 100 microseconds.Various other embodiments may perform such switching of a light emittingdiode in less than 25 microseconds. Various other embodiments mayperform such switching of a light emitting diode in greater than 100microseconds. Timing for such switching of a light emitting diode froman on-state to an off-state may differ from timing for such switching ofa light emitting diode from an off-state to an on-state.

In various embodiments, light emitting diodes 204 206 208 may bemonochromatic, and a driving current (e.g., a maximum driving current)supplied to a light emitting diode may be specific to the color outputof the light emitting diode. For example, in one embodiment, red lightemitting diodes may be driven with a driving current of approximately 10amperes, blue light emitting diodes may be driven with a driving currentof approximately 11 amperes, and green light emitting diodes may bedriven with a driving current approximately 17 amperes. In various otherembodiments, other driving currents (maximum or otherwise) for variouscolor output light emitting diodes may be used. In various alternativeembodiments, a driving current (e.g., a maximum driving current)supplied to a light emitting diode may not be specific to the coloroutput of the light emitting diode.

FIG. 3 illustrates some elements of a light emitting diode drivercircuit, in accordance with one embodiment. As illustrated, lightemitting diode driver circuit 300 may receive input power from voltagesupply Vbulk 302. In some embodiments, power processing block 104 mayinclude buck converter 352. Buck converter 352 may include Schottkydiodes 348 350, MOSFET 344, and inductor 346. While buck converter 352includes Schottky diodes, other embodiments may include other types ofdiodes and may include a different number of diodes. While buckconverter 352 includes diodes, other embodiments may not include anydiodes. While buck converter 352 includes a MOSFET, other embodimentsmay include a different mechanism for implementing a switch. Vbulk 302may be conditioned with capacitors 304 306 308 310 to remove anyalternating current component in Vbulk 302.

In various embodiments, current sensing circuit 112 may include currentsensing resistor 312, and differential amplifier 354. Differentialamplifier 354 may include operational amplifier 330, and resistors 324326 328 332, arranged as shown. Values of resistors 324 326 328 332 areapplication dependent, and may vary from implementation toimplementation. Differential amplifier 354 may provide someamplification by operational amplifier 330 having a gain greater thanone. Current sensing circuit 112 may include a gain stage, which mayinclude operational amplifier 338 and resistors 334 336, arranged asshown. Values of resistors 334 336 are application dependent, and mayvary from implementation to implementation. Pulse width modulationcontroller 340 may be an integrated circuit including error amplifiercircuit 114, pulse width modulation circuit 118, and clock circuit 120of FIG. 1. Voltage reference circuit 116 in FIG. 3 may output a voltagesupplied by a voltage source represented as Vcontrol 342. In variousembodiments, Vcontrol 342 may be fixed, while in various otherembodiments, Vcontrol 342 may be modifiable. Pulse width modulationcontroller 340 may include external components as may be specified bythe manufacturer or otherwise known in the art to facilitate itsoperation.

Capacitor 322 may act to reduce ripple current in the driving currentbeing supplied to light emitting diodes 316 318 320. While theembodiment in FIG. 3 shows three light emitting diodes coupled inseries, other embodiments may include a greater or lesser number oflight emitting diodes, serially or non-serially coupled together.

Pulse width modulation controller 340 may be electrically coupled toMOSFET 344, and may supply a control signal to MOSFET 344. The pulsewidth of the control signal supplied to MOSFET 344 may be at leastpartially modulated to compensate for the ripple sensed in the drivingcurrent being supplied to the one or more light emitting devices. Invarious embodiments, the ripple may include at least approximately fiftykilohertz of oscillation. In various embodiments, the ripple may fallwithin the range of approximately between twenty kilohertz and onemegahertz.

Light emitting diode driver circuit 300 may be capable of turning itsone or more light emitting diodes from an on-state to an off-state orfrom an off-state to an on-state, including settling at the desiredamplitude, in the time range of approximately 25 to 100 microseconds.Various other embodiments may perform such switching of a light emittingdiode in less than 25 microseconds. Various other embodiments mayperform such switching of a light emitting diode in greater than 100microseconds. Timing for such switching of a light emitting diode froman on-state to an off-state may differ from timing for such switching ofa light emitting diode from an off-state to an on-state.

In various embodiments, current sensing resistor 312 may be the currentsensing portion of current sensing circuit 112. Current sensing resistor312 may be coupled in series with the one or more light emitting diodes,a node 314 of current sensing resistor 312 being coupled to the anode ofthe first light emitting diode connected in series with other lightemitting diodes. Such an embodiment of current sensing circuit 112 mayprovide good noise immunity.

In various embodiments, light emitting diodes 316 318 320 may bemonochromatic, and a driving current (e.g., a maximum driving current)supplied to a light emitting diode may be specific to the color outputof the light emitting diode. For example, in one embodiment, red lightemitting diodes may be driven with a driving current of approximately 10amperes, blue light emitting diodes may be driven with a driving currentof approximately 11 amperes, and green light emitting diodes may bedriven with a driving current approximately 17 amperes. In various otherembodiments, other driving currents (maximum or otherwise) for variouscolor output light emitting diodes may be used. In various alternativeembodiments, a driving current (e.g., a maximum driving current)supplied to a light emitting diode may not be specific to the coloroutput of the light emitting diode.

FIG. 4 illustrates projection system 400, which includes light emittingdevice based illumination source 402 electrically coupled to lightemitting device driver circuit 404, in accordance with an embodiment ofthis invention. Light emitting device driver circuit 404 may be coupledto power supply 406. Light emitting device based illumination source 402may be optically coupled to light valve 410. Light emitting device basedillumination source 402, driven by light emitting device driver circuit404, may sequentially provide light valve 410 with incident light 416 ofa number of constituent colors by pulsing one or more light emittingdevices of each constituent color. The constituent colors may be thoughtof as individual colors that, when combined in the appropriate amounts,create an object color for the image pixel. In one embodiment theconstituent colors may include red, green and blue, however, alternativeembodiments may additionally or alternatively employ many other colors,including white and other color combinations. In various embodiments,light emitting device driver circuit 404 may be one of the earlierdescribed light emitting diode circuits.

Light valve 410 may be coupled to controller 408. Controller 408 may beadapted to receive image signals from image signal source 414 and totransmit light valve control signals to light valve 410. In variousembodiments, image signal source 414 may include, but is not limited to,a personal or laptop computer, a personal data assistant (PDA), acellular phone, a digital versatile disk (DVD) player, a set-top box, anintegrated television tuner, a video camera, or any other sourcesuitable for transmitting image signals to projection system 400.Projection system 400 may be implemented in a variety of differentapplications including, but not limited to, games, movies, television,advertising and data display.

In various embodiments, image signal source 414 may represent the sourceof an image signal being transmitted over a communications network toprojection system 400. Such a network may include one or more of thefollowing example communications networks: a personal area network(PAN), a local area network (LAN), a wide area network (WAN), ametropolitan area network (MAN), the Internet, etc. In variousembodiments, such networks may in whole or in part comprise of landlineand wireless links.

Controller 408 may generate image frames from an analog image signalbased on a desired frame rate. This process may be omitted if theincoming image signal is a digital signal, as the image frames shouldalready be defined. However, there may be some instances where a framerate conversion of the source video signal may be desired. Based on thedesired object color of each image pixel, controller 408 may generateand route pixel control data for the individual light valve pixels inorder to facilitate the modulation of incident light 416 from lightemitting device based illumination source 402 into image bearing light418. Controller 408 may be electrically coupled to light emitting devicedriver circuit 404 to in order to synchronize pixel control data withlight emitting device pulse sequences. Image bearing light 418 may thenbe passed on to projection optics 412, which may facilitate finalimaging. In various embodiments, final imaging may occur on a screen, adisplay, or some other suitable device. In various embodiments,projection system 400 may be integrated in a projector. In variousembodiments, projection system 400 may be integrated in a projectiontelevision. In various embodiments, projection system 400 may beintegrated in another device that may make use of image or videoprojection. Projection optics 412 may include a projection lens, animaging lens, and many other optical components known in the field.

Thus, it can be seen from the above description, circuits for drivinglight emitting devices being used as illumination sources of aprojection system, and projection subsystems and systems so equipped,have been described. While the present invention has been described interms of the foregoing embodiments, those skilled in the art willrecognize that the invention is not limited to the embodimentsdescribed. Other embodiments may be practiced with modification andalteration within the spirit and scope of the appended claims.Accordingly, the description is to be regarded as illustrative insteadof restrictive.

1. A driving circuit comprising: a current sensing circuit coupled toone or more light emitting devices, the current sensing circuit adaptedto sense driving current being supplied to the one or more lightemitting devices; a pulse width modulation circuit coupled to thecurrent sensing circuit and adapted to modulate a pulse width of acontrol signal based at least in part on an output of the currentsensing circuit, the control signal being employed to adjust the drivingcurrent being supplied to the one or more light emitting devices; and abuck converter coupled to the one or more light emitting devices, thebuck converter adapted to switch the driving current being supplied tothe one or more light emitting devices.
 2. The driving circuit of claim1, wherein at least one of the one or more light emitting devicescomprises a selected one of a light emitting diode and a laser diode. 3.The driving circuit of claim 1, wherein the driving circuit furthercomprises an error amplifier circuit coupled to the current sensingcircuit, and the current sensing circuit is adapted to supply a firstvoltage to the error amplifier circuit, the first voltage being relatedto the driving current being supplied to the one or more light emittingdevices.
 4. The driving circuit of claim 3, wherein the error amplifiercircuit is further supplied with a second voltage, and the secondvoltage is used at least in part to specify the amplitude of the drivingcurrent being supplied to the one or more light emitting devices.
 5. Thedriving circuit of claim 4, wherein the second voltage is a variablevoltage that is used at least in part to specify the amplitude of thedriving current being supplied to the one or more light emittingdevices.
 6. The driving circuit of claim 1, wherein the driving currentbeing supplied to the one or more light emitting devices is greater thanor equal to approximately 1 ampere.
 7. The driving circuit of claim 1,wherein the pulse width modulation circuit modulates the pulse width ofthe control signal to compensate for a ripple sensed in the drivingcurrent being supplied to the one or more light emitting devices, theripple including at least approximately fifty kilohertz of oscillation.8. The driving circuit of claim 1, wherein the pulse width modulationcircuit modulates the pulse width of the control signal to compensatefor a ripple sensed in the driving current being supplied to the one ormore light emitting devices, the ripple being approximately betweentwenty kilohertz and one megahertz.
 9. The driving circuit of claim 1,wherein the one or more light emitting devices are coupled to each otherin series, the anode of one light emitting device being at a highervoltage potential than the anodes of the other light emitting devicescoupled in series; and a current sensing portion of the current sensingcircuit is coupled in series with the one or more light emittingdevices, a node of the current sensing portion of the current sensingcircuit being coupled to the anode of the first light emitting device.10. The driving circuit of claim 1, wherein the one or more lightemitting devices are monochromatic and a driving current supplied to theone or more light emitting devices is specific to the color output ofthe light emitting devices.
 11. The driving circuit of claim 1, whereinthe current sensing circuit comprises at least one current sensingresistor.
 12. The driving circuit of claim 1, wherein the currentsensing circuit comprises at least one operational amplifier.
 13. In aprojection system, a method of controlling a driving circuit comprising:sensing driving current being supplied to one or more light emittingdevices; modulating the pulse width of a control signal to adjust thedriving current based at least in part upon the sensed driving current;and switching the driving current being supplied to the one or morelight emitting devices at least in part by use of a buck converter. 14.The method of claim 13, wherein at least one of the light emittingdevices comprises a selected one of a light emitting diode and a laserdiode.
 15. The method of claim 13, wherein the modulating of the pulsewidth of the control signal is at least partially performed tocompensate for a ripple sensed in the driving current being supplied tothe one or more light emitting devices, the ripple including at leastapproximately fifty kilohertz of oscillation.
 16. The method of claim13, wherein the modulating of the pulse width of the control signal isat least partially performed to compensate for a ripple sensed in thedriving current being supplied to the one or more light emittingdevices, the ripple being approximately between twenty kilohertz and onemegahertz.
 17. The method of claim 13 further comprising supplying acurrent to the one or more light emitting devices based on the outputcolor of the light emitting devices, the one or more light emittingdevices being monochromatic.
 18. A system comprising: an illuminationsource, the illumination source including a light emitting device; and adriver circuit coupled to the light emitting device, the driver circuitincluding: a current sensing circuit adapted to sense the current beingsupplied to the light emitting device; a pulse width modulation circuitcoupled to the current sensing circuit, the pulse width modulationcircuit being adapted to modulate the pulse width of a control signal toadjust the current being supplied to the light emitting device based atleast in part upon an output of the current sensing circuit; and a buckconverter coupled to the light emitting device, the buck converteradapted to switch the driving current being supplied to the lightemitting device.
 19. The system of claim 18, further comprising acontroller, adapted to receive an image signal and to output a lightvalve control signal representing an image for a frame; and a lightvalve, coupled to the controller and optically coupled to theillumination source, the light valve being adapted to modulate anincident light produced by the illumination source into image bearinglight based on the light valve control signal.
 20. The system of claim19, further comprising projection optics, optically coupled to the lightvalve to receive the image bearing light and to project the image. 21.The system of claim 20, wherein the controller is further adapted toreceive and project a series of images as video.
 22. The system of claim18, further comprising an image signal source, coupled to thecontroller, to output the image signal.
 23. The system of claim 18,wherein the system comprises one of a group consisting of a projectorand a projection television.